Wideband Code Division Multiple Access (WCDMA) is a mobile radio access network standard specified by a 3rd Generation Partnership Project (3GPP) and used in third generation wireless data/tele-communication systems.
Soft handover is an important feature in all WCDMA systems. With soft handover, User Equipment Nodes (UEs) are effectively connected to several base stations at the same time. This improves reliability, since a UE that moves from one cell to another establishes a link to the new cell, before the link to the old cell is torn down. The way that soft handover is combined with transmit power control also limits uplink (UL) interference in soft handover scenarios.
An example WCDMA system 100 is shown in FIG. 1. The system 100 includes a UE 110 that communicates with two base stations (also referred to as “Node Bs” and “network nodes” herein) 120a and 120b through a radio air interface. The base stations 120a-b are controlled by a radio network controller (RNC) 130 and connected to a core network 140.
Because WCDMA and High Speed Packet Access (HSPA) use common frequencies in all cells, a UE located at a border of one cell will cause uplink interference to neighboring cell(s). This interference can be particularly problematic because the UE located at the cell border (cell edge) will typically be transmitting near an upper range of its UL transmit power.
Soft handover can be used to reduce uplink interference to the neighboring cell(s). UEs at the border between two cells are connected to both (several in the general case) cells. The UEs therefore transmit data to both cells and receive data from both cells.
Transmit power control (or simply power control) is another important operation in WCDMA UL, since the channels within one cell are non-orthogonal; a transmission from one UE can strongly interfere with a transmission from another UE. In a WCDMA system, the uplink and downlink communications are power controlled. The UE signals to a base station how it shall regulate its downlink transmission power. In a similar way the base station signals to the UE how it shall regulate its uplink transmission power. A new transmit power control (TPC) command can be signaled every slot (e.g., 1500 TPC commands per second). Accordingly, at 1500 times per second, each UE can be commanded to either increase or decrease its transmit power by a predetermined step.
The base station can control the UL packet error rate performance and UL interference by controlling the transmission power of the UE. As the UE increases its transmission power the experienced signal to interference ratio at the base station will in general increase. An increased signal to interference ratio will result in a lower packet error rate. In this way the base station can tune the uplink packet error rate.
In soft handover, a UE is receiving power control commands from more than one cell. Based on the received signal, each cell commands the UE to either increase or decrease its transmit power. The UE thus receives several, possibly conflicting, transmit power commands. The power control commands from the cells are combined to decide if the UE should either increase or decrease its transmit power. To combine the power control commands, the UE follows an operational rule that if any power control commands contain a DOWN request, the UE reduces its transmit power.
A UE transmits data and control information on physical channels that can include Dedicated Physical Control CHannel (DPCCH), Enhanced-DPCCH (E-DPCCH), Enhanced Dedicated Physical Data CHannel (E-DPDCH), and High-Speed DPCCH (HS-DPCCH). The DPCCH transmits pilot bits that are known by the base station and also Layer 1 control information. The pilot bits are used as a reference by the base station to estimate the radio channel conditions (e.g. searcher, channel estimation, frequency offset estimation, and signal to interference ratio). The E-DPCCH transmits control information related to the enhanced dedicated physical data channel. The E-DPDCH transmits the data bits.
FIG. 2 illustrates graphs of transmission power levels and associated Transmission Power Control (TPC) commands that may be transmitted from a base station to a UE and, vice versa, from the UE to the base station to control the transmission power levels in the downlink and uplink directions. The base station measures the UL signal-to-interference ratio (SIR) on the DPCCH and compares it with a target value of the SIR. When the measured SIR is above the target SIR, the base station signals to the UE to decrease its transmission power. When the measured SIR is below the target SIR, the base station signals to the UE to increase its transmission power. For UEs capable of transmitting enhanced uplink, the UL-TPC (Up Link Transmission Power Control) commands are signaled to the UE on the downlink (DL) channel F-DPCH (Fractional Dedicated Physical CHannel).
In a similar manner the UE measures the quality of the F-DPCH that it receives from the base station. When the quality is sufficient, the UE signals to the base station that it can decrease the transmission power on the F-DPCH. When the quality is not sufficient, the UE signals to the base station to increase the transmission power on the F-DPCH. The DL-TPC commands are sent to the base station on the uplink channel DPCCH. The transmission power level of the E-DPCCH and the E-DPDCH may be controlled in response to a power offset relative to the transmission power level of the DPCCH.
The TPC commands control the UE transmit power for DPCCH (PDPDCH). The UE controls the transmit power levels for the E-DPCCH (PE-DPCCH), the E-DPDCH (PE-DPDCH), and the HS-DPCCH (PHS-DPCCH) in response to a predefined power offset relative to the transmission power level of the DPCCH (PDPDCH), as follows:PDPDCH=βDPDPCCH PE-DPCCH=βECPDPCCH PE-DPDCH=βEDPDPCCH PHS-DPCCH=βHSPDPCCH where all the β parameter values are independent of (not controlled in response to) the TPC commands. The β parameter values are computed based on quantized amplitude ratios, which are translated from ΔACK, ΔNACK, and ΔCQI signaling. Computation of β parameter values is explained in 3GPP TS 25.213 V10.0.0 (2010-09), Sect. 4.3.1.2, and translation of ΔACK, ΔNACK, and ΔCQI signaling into β parameter values is explained in 3GPP TS 25.214 V10.0.0 (2010-09), Sect. 5.1.2.5A.
FIG. 3 illustrates a UE 110 that is connected to two base stations 120a and 120b in a heterogeneous network. One of the base stations 120b is a low power node (e.g., pico node) having significantly lower transmission power than the other base station 120a (e.g., macro node). Cell selection is typically based on downlink received power, with the illustrated UE 110 typically connected to the base station 120a or 120b from which it receives the highest transmit power, including effects of the different base station transmission powers.
This leads to a cell area surrounding the low power base station 120b where the high power base station 120a is selected, but where the path loss is lower toward the lower power base station 120b. In the uplink direction, where the transmit power is the same, it would be better for the UE 110 to be connected to the low power base station 120b. 
If several base stations are received with similar transmit powers, the UE 110 enters soft handover. The above described operational rule for combining power control commands received from different base stations is intended to ensure that data reaches one of the cells involved in a soft handover. For the DPDCH, E-DPCCH and E-DPDCH, it is indeed enough that the transmission reaches one of the cells. However, the HS-DPCCH must reach one specific cell out of the cells involved in the soft handover, the so-called serving HS cell. The serving HS cell is the cell from which the high-speed downlink shared channel (HS-DSCH) is transmitted. The HS-DPCCH carried channel quality reports (CQIs) and ACK/NACKs that must reach the serving HS cell within a short delay (in the order of a few milliseconds).
With the current TPC command combining rule, the DPCCH transmit power may be reduced so that the HS-DPCCH is only received in the non-serving cell. To help alleviate the problem, the parameter βHS may be increased by a certain amount when the UE 110 enters soft handover. However, such an increase may be unnecessary in some cases, resulting in too high of an interference level (an unnecessary interference level).
However, more importantly, in the context of the heterogeneous network of FIG. 3, where the UE 110 communicates with the low power base station 120b and the high power base station 120a, and the UE 110 is connected to the base station with the strongest received signal, the cell-border can be quite close to the low power base station 120b. In this case, the low power base station 120b will receive a correspondingly very strong signal from the UE 110, and it will subsequently order the UE 110 to reduce it transmit power quite significantly. As a result, the signal received at the high power base station 120a (which may still be serving HE cell), will be quite weak, and using a fixed, large offset βHS will be insufficient for the high power base station 120a to properly receive the signal from the UE 110.
Another situation where this problem is made worse is when Coordinated Multi-Point (CoMP) is deployed. When operating with CoMP, the base stations (e.g., 120a and 120b) involved in the soft handover will transmit different signals over the HS-DSCH, and the individual HS-DPCCH transmissions must reach both base stations (e.g., 120a and 120b).
The approaches and presently recognized problems described above in this section could be pursued, but are not necessarily approaches and/or problems that have been previously conceived or pursued. Therefore, unless otherwise clearly indicated herein, the approaches and problems described above in this section are not prior art to claims in any application claiming priority from this application and are not admitted to be prior art by inclusion in this section.